Figure 4.

Coexpression of genes in SMC cells. Whole-mount in situ hybridization (WISH) analysis of examples of signaling and transcription factor genes
identified in this screen. FGF20 (Sp-FGF9/16/20), the only fibroblast growth factor present in the sea urchin genome, is expressed
in primary mesenchyme cells (PMCs) and around the apical organ during gastrulation,
whereas two receptors identified in this screen are expressed in adjacent secondary
mesenchyme cells (SMCs; FGFR3, blastula stage) and in SMCs and the central apical region (FGFR1, left blastula, right gastrula). The transcription factors Prox1, Tbx6, Six1, Sox17, and snail are expressed in SMCs during gastrulation, as is a PKCdelta1 gene. In all pictures the animal sides of the embryos is located towards the top.
Annotated images of additional stages can be found in the WISH database [22].